Module tokio::sync::watch

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Expand description

A single-producer, multi-consumer channel that only retains the last sent value.

This channel is useful for watching for changes to a value from multiple points in the code base, for example, changes to configuration values.


channel returns a Sender / Receiver pair. These are the producer and consumer halves of the channel. The channel is created with an initial value. The latest value stored in the channel is accessed with Receiver::borrow(). Awaiting Receiver::changed() waits for a new value to be sent by the Sender half.


use tokio::sync::watch;

let (tx, mut rx) = watch::channel("hello");

tokio::spawn(async move {
    while rx.changed().await.is_ok() {
        println!("received = {:?}", *rx.borrow());



Sender::is_closed and Sender::closed allow the producer to detect when all Receiver handles have been dropped. This indicates that there is no further interest in the values being produced and work can be stopped.

The value in the channel will not be dropped until the sender and all receivers have been dropped.

Thread safety

Both Sender and Receiver are thread safe. They can be moved to other threads and can be used in a concurrent environment. Clones of Receiver handles may be moved to separate threads and also used concurrently.




  • Creates a new watch channel, returning the “send” and “receive” handles.